Space is full of ionized matter travelling at significantly speeds (mostly protons and electrons in plasma state, but also heavier ions like He, Fe and others). We usually call this matter: ionizing radiation. The principal constituent of energy in this particles is kinetic energy.
As radiation passes through matter, it may undergo changes in its composition and energy by means of transformation or exchanging energy with other particles. When it arrives to a body it could pass through it and/or it could interact with it. The effects can be damaging to beings and objects and may result in death, disease, impairment, destruction and improper function. Active and passive shielding are used to stop or reduce the effects of radiation over an specific target. In the case of active shielding, the desired result is to stop or deflect away the incoming particle from the protected target. For active shielding, mainly electrostatic fields and magnetic fields have been proposed. Active shielding has been use to some extent, yet the size, energy consumption and mass of these systems has reduced their use to a very specific applications. Their effectiveness depends on the strength of the field, the distance over which the field acts, and the rigidity of the radiation particle.
The intrinsic problems of active electrostatic shielding are the relatively low field strength, the need for active power to run it, and the requirement for huge mass and volume allowances. Usually active electrostatic shielding requires dimensions hundreds of meters and mass of several tons of steel, making it unpractical.
Magnetic active shielding also requires heavy equipment, superconductors to build magnets and is highly power demanding.
Ionizing radiation shielding schemes have, so far, relied mainly on passive shielding. For radiation formed out of non charged particles are—so far—the only way to mitigate or stop the effects.
Traditionally, passive shielding objective has been to absorb into the shield the energy that would have damage the protected target. Passive shielding is widely used due to its simplicity, the lack of constant energy requirements and its foreseeable performance.
Some attempts have been made in the past to improve passive shielding by including magnetic particles within the material, yet the results have not shown significant improvements. Also, when using magnetic elements to alter the direction of the incoming particle, the behaviour of the shield is asymmetric. Depending the trajectory angle of the incoming particle, the magnetic field could deflect radiation
towards the target instead of away from it. This effectively limits its use to cases where the radiation particles direction can be established in advance.
Passive shields can generate secondary radiation such as other particles, gamma rays and neutrons that may not be diverted. Passive shielding attenuates radiation by slowing and moderating it, resulting in the deposition of the radiation's energy in the passive shield and possibly resulting in the complete capture and absorption of the radiation. The success of passive shielding depends upon the stopping power of the shield. This value is a function of the number of interactions with atoms or their parts. The probability for interactions is dependent on the length of the path travelled by the ionizing radiation through the shield. The longer the path that the particle travels, greater is the opportunity for the passive shield to moderate and absorb the radiation. The main drawback of passive shielding is the necessity of very big, dense and/or heavy shields to guaranty enough interactions between the radiation particle and the shield. Also, for certain materials the production of secondary radiation (neutrons, electromagnetic radiation and charged particles) can significantly worsen the effect on the target.